The present invention relates to an electrostatic actuator which is manufactured using an MEMS (Micro Electro-Mechanical Systems) technique and, more particularly, to an electrostatic actuator which is applied to a micro switch for turning on or off a wide band signal frequency of from DC to several hundreds of GHz, a light switch for switching the direction of a light signal according to the inclination of the mirror, a scanner for switching the direction of a relevant wireless antenna, etc.
A conventional technique will now be explained by taking up as an example thereof a technique and device that are described in a treatise entitled xe2x80x9cA Micro-Machined Microwave Antenna Integrated with its Electrostatic Spatial Scanningxe2x80x9d (Proceedings of IEEE Micro Electro Mechanical Systems, Nagoya, pp. 84-89, 1997), pronounced in the IEEE 10th Micro Electro Mechanical Systems International Conference by Dominique Chauver et al. of Tokyo Univ. LIMMS/SNRS-II.
A perspective view of this device is illustrated in FIG. 1. In this device, a quartz substrate 610 is machined to form a torsional vibration plate 611 and springs 613 that support both ends of the vibration plate 611. On the upside surface of the torsional vibration plate 611, there is provided an upper electrode 612 consisting of a chrome/gold material, and this upper electrode 612 is electrically connected to a contact pad 614 through the intermediary of a wiring 615, on the other hand, with respect to a silicon substrate 620, there is formed an inclination structure 621. Chauver et al. formed the inclination structure 621 having two inclined surfaces the angle of inclination of that is 35.3xc2x0 by performing anisotropic wet etching with respect to a silicon substrate having a (110) Si crystal face. They formed two electrode patterns, lower electrodes 622a and 622b each made of chrome, respectively, on those two inclined surfaces. These lower electrodes 622a and 622b are respectively electrically connected to contact pads 624a and 624b. These quartz substrate 610 and silicon substrate 620 are bonded together in the way of being aligned with each other such that the torsional vibration plate 611 may be located over the inclination structure 621 (provided, however, that the method of bonding is not described).
Applying a voltage between the upper electrode 612 and the lower electrode 622a or 622b, due to the electrostatic attracting force an attractive force that acts toward the substrate (downside) occurs in the torsional vibration plate 611. For this reason, the springs 613 are torsion-deformed (twisted), with the result that the torsional vibration plate 611 rotates about the springs 613 and gets inclined. By varying the voltage applied between the upper electrode 612 and the lower electrode 622a or 622b, it is possible to adjust the rotation angle of the torsional vibration plate 611. Also, by selecting which of the lower electrodes 622a and 622b a voltage is applied to, it is possible to change the rotation direction of the torsional vibration plate 611.
In this conventional technique, the application of the device to an antenna that changes the transmission direction or reception direction of a radio signal by varying the rotation direction of the torsional vibration plate 611 was stated. What is particularly noticeable is that by forming the lower electrode into an inclination structure it is possible to decrease the voltage that is applied. This is based on the principle that, since an electrostatic attracting force decreases in inverse proportion to the square of the distance between two structures, if the device can be designed so as to make small the distance between the upper electrode and the lower electrode, the voltage that is applied can be made small. When the rotation angle of the torsional vibration plate 611 is zero, a large electrostatic attracting force occurs between the upper electrode region and the lower electrode 622a/622b region the lower electrode portion of that is provided at the position that is near the apex of the inclination structure 621. As the torsional vibration plate 611 rotates, a large electrostatic attracting force also goes on occurring in the other regional portion, as well, of the lower electrode 622a/622b. If the lower electrode 622a/622b is provided on a flat surface having no inclination structure 621, since the distance between the upper electrode and the lower electrode is large, a high level of voltage is needed for the purpose of rotating the torsional vibration plate 611. Although Chauver et al. do not concretely state that effect of the inclination structure, calculating the electrostatic attracting force in relation to the inclination structure of 35.3xc2x0, it proved that the voltage that is applied can be decreased approximately 30% with respect to the flat structure.
Also, although Chauver et al. do not state, the second effect of the inclination structure 621 is to make more likely to occur the rotational movement about the springs 613 of the torsional vibration plate 611. When applying a voltage between the upper electrode 612 and the lower electrode 622a/622b, a force that acts toward the lower electrode occurs in the upper electrode 612. However, in a case where the rigidity of the bending deformation of the springs 613 is smaller than the rigidity of the rotation (torsion), the tendency to deform toward the silicon substrate 620 side perpendicularly with respect thereto becomes more likely to occur than the tendency to rotate. The inclination structure 621 plays the role of preventing that perpendicular deformation and causing only the rotational movement alone to occur in the torsional vibration plate 611.
FIGS. 2A to 2D are sectional views illustrating a method of manufacturing the structure on the silicon substrate side according to the above-described conventional technique. A silicon nitride film 72a and a silicon nitride film 72b are deposited on both surfaces, respectively, of a silicon substrate 71 the (110) Si crystal face of that serves as a principal surface by using a low-pressure vapor phase epitaxy (LP-CVD). And, with respect to one surface of them, patterning of the nitride film 72a is performed using a photolithography technique (the same figure A). This substrate is put into a 33% solution of KOH, thereby performing anisotropic etching with respect to the silicon substrate 71. As a result of this, an inclination structure 73 having an inclination of 35.3xc2x0 with respect to the flat surface is formed (the same figure B). Subsequently, on the surface of the silicon substrate having this inclination structure 73, by sputtering, a silicon oxide film is deposited. A metal mask 76 is disposed on this resulting substrate, then chrome is deposited. At this time, through the openings formed in the metal mask 76, the chrome is deposited on the inclination structure, thereby a lower electrode 75 can be formed (the same figure C). Thereafter, again, by sputtering, a silicon oxide film 77 is deposited on the chrome lower electrode 75 (the same figure D). Finally, a torsional vibration plate formed by machining a quartz substrate is bonded onto that silicon substrate 71, thereby the device illustrated in FIG. 1 is manufactured.
In this conventional technique, the torsional vibration plate had a dimension of 1xc3x972xc3x970.1 mm. Especially, for the reason why the torsional vibration plate having a width as great as 2 mm is designed to be inclined xc2x110, it was necessary to construct so that the height of the inclination structure may be equal to or more than 175 xcexcm. For forming the lower electrode pattern on the substrate having a level difference that is as great as that height, Chauver et al. adopted the chrome deposition method utilizing a metal mask 76 such as that illustrated in FIG. 2C. However, due to the existence of a clearance between the metal mask 76 and the inclination structure 75, it is difficult to form the lower electrode 75 with the dimensions as designed, and at the position as designed. This is because, since the chrome particles that have gone out from the target of the deposition device collide with the substrate at a certain angle of spread, the fact that the distance between the substrate and the target varies, if happening, causes a shift of the collision position from their proper one. In this conventional technique, as the position gets shifted from the apex of the inclination structure, the distance between the inclined surface and the target gets increased. This raised the problem that the pattern became different from the metal mask. In the electrostatically driven actuator, its characteristic is very highly sensitively affected by the configurations of the upper/lower electrodes and positional relationship therebetween. On that account, when evaluating the torsion angle in relation to the driving voltage by using the device according to the conventional technique, it proved that the characteristic greatly varied between the devices.
The problem that the lower electrode pattern cannot be formed faithfully according to the mask can not be solved even when using the method of forming a resist pattern directly with respect to the inclination structure. This is because, in this case, transferring the photo-mask pattern faithfully with respect to the inclined surface of the inclination structure is very difficult on account of a limitation existing when accurately obtaining the focal distance of the optical system of a relevant exposure device. Also, it is difficult to evenly coat the resist with respect to the inclination structure, too.
For the above-described reasons, despite the merit of the inclination structure being able to decrease the voltage that is applied, because it is difficult to accurately form the electrode pattern on the inclination structure, there was the problem that it was difficult to manufacture a device that was reliable and the characteristic of that was uniformly qualified. For this reason, it was not only impossible to supply a uniformly qualified quality of the products in large amount as the mass-production goods but was it difficult to utilize the inclination structure with respect to the use purposes including a high-function antenna required to have array-allocated a large number of the actuators, a light switch for switching a number of signals, and an electrical switch serving the same purpose. These were serious problems.
The present invention has been made in view of the above-described problems and has an object to provide an electrostatic actuator that enables manufacturing electrostatic actuator devices which are reliable as the mass-production goods, and the characteristics of which are uniformly qualified, while they have the merit of the inclination structure.
To attain the above object, a first feature of the present invention is that, in an electrostatic actuator comprising an upper structure that is connected, via an arm, to a supporting base provided on a substrate and is supported in a space existing over the substrate, a lower structure that is provided in a substrate position in such a way as to oppose the upper structure, an inclination structure that is provided with respect to either one of the upper structure and the lower structure so as to make small the distance between the upper structure and the lower structure, and one or more electrodes that are provided with respect to the other structure in corresponding relationship to the inclination structure, by a voltage being applied between the electrode and a structure having the inclination structure, the upper structure is inclined toward the lower structure side.
A second feature of the present invention is that the electrode is provided with respect to a flat surface of the other structure.
A third feature of the present invention is that an insulating film is provided on the flat surface of the other structure; and, on the insulating film, the electrode is formed using an electrically conductive material.
A fourth feature of the present invention is that the other structure having the flat surface is constructed of a semiconductor material and the electrode is formed on the surface of this structure by using a material having a conductivity type opposite to that of the semiconductor material.
A fifth feature of the present invention is that the electrode is provided on the opposite surface of the mutually opposing surfaces of the upper structure and the lower structure.
A sixth feature of the present invention is that the substrate is of a glass substrate.
A seventh feature of the present invention is that each of the supporting base and the arm is constructed such that two pieces thereof constitute one set; the arm has the function of a torsion spring and the upper structure is supported by the arms; and there are provided the two or more electrodes, so that, by switching the electrode to which a voltage is applied, the direction in which the upper structure is inclined is controlled.